Conventionally, as a technique for correcting an image, a method disclosed in Japanese Published Patent Application No. 2002-172423 (Pages 14˜16, FIGS. 1˜4) has been generally known. This method includes calculating luminous values of pixels of an input image, creating a luminous histogram illustrating the total sum of pixels having the same luminous value, and subjecting all of the pixels to a correction process that optimizes the shape of the created luminous histogram.
The conventional method will be described with reference to FIG. 15.
FIG. 15 is a block diagram illustrating a conventional image quality correction apparatus.
As shown in FIG. 15, the conventional image quality correction apparatus comprises an image pickup element 1401, an A/D converter 1402, a luminous signal calculation circuit 1403, a luminous histogram formation circuit (luminous distribution formation circuit) 1404, a correction amount calculation circuit 1405, and an image quality correction circuit (image signal correction circuit) 1406. In the image quality correction apparatus, the correction amount calculation circuit 1405 comprises a barycenter calculation circuit 1407, a distribution range calculation circuit 1408, a target value storage circuit 1409, and a correction table formation circuit 1410.
The luminous signal calculation circuit 1403 calculates luminous values of image data outputted from the A/D converter 1402, and the luminous histogram formation circuit 1404 forms a luminous histogram of the input image by using the luminous values calculated by the luminous signal calculation circuit 1403.
Further, the barycenter calculation circuit 1407 calculates a barycenter position of a low luminous part from the luminous histogram formed by the luminous histogram formation circuit 1404, and the distribution range calculation circuit 1408 calculates the maximum value and the minimum value on the input luminous level axis from the luminous histogram formed by the luminous histogram formation circuit 1404.
The target value storage circuit 1409 holds a target value of luminous correction, and the correction table formation circuit 1410 forms a correction table in which the luminous distribution is enlarged and, simultaneously, the barycenter position matches the target value stored in the target value storage circuit 1409, on the basis of the barycenter position which is calculated by the barycenter calculation circuit 1407, and the maximum and minimum values on the input luminous level which are calculated by the distribution range calculation circuit 1408.
The image quality correction circuit 1406 performs gradation correction with reference to the correction table formed by the correction table formation circuit 1410.
Next, the operation of the conventional image quality correction apparatus will be described.
An image signal outputted from the image pickup element 1401 is converted into image data by the A/D converter 1402, and the image data is output to the luminous signal calculation circuit 1403 and to the image quality correction circuit 1406.
The image data outputted from the A/D converter 1402 is input to the luminous signal calculation circuit 1403, and the luminous values of the respective pixels constituting the image data are calculated, and thereafter, a luminous histogram is formed by the luminous histogram formation circuit 1404.
The luminous histogram is output to the barycenter calculation circuit 1407 and to the distribution range calculation circuit 1408. On the basis of luminous histogram of the input image, the barycenter calculation circuit 1407 calculates the barycenter position of the low luminous part, and the distribution range calculation circuit 1408 calculates the maximum and minimum values on the input luminous level axis.
Then, the barycenter position of the low luminous part calculated by the barycenter calculation circuit 1407, and the maximum and minimum values on the input luminous level axis calculated by the distribution range calculation circuit 1408 are input to the correction table formation circuit 1410, and a correction table is formed so that the luminous distribution is enlarged and, simultaneously, the barycenter position matches the luminous correction target value that is stored in the target value storage circuit 1409.
Then, the image data outputted from the A/D converter 1402 and the correction table outputted from the correction table formation circuit 1410 are input to the image quality correction circuit 1406, whereby the image data is subjected to gradation correction on the basis of the correction table, thereby automatically carrying out optimum gradation correction for the input image.
In the conventional construction, however, when a character area, such as subtitles of a movie, exists in an input image, the luminous values of the characters are undesirably counted during formation of the luminous histogram, although only the image data in the image area excluding the character area should be corrected. Therefore, a luminous histogram corresponding to only the image area of the input image cannot be formed, and an accurate amount of correction cannot be calculated. As a result, optimum correction for the input image cannot be carried out.
Further, in an apparatus capable of selecting presence or absence of subtitles, such as a DVD player, since the shape of the luminous histogram varies depending on presence/absence of subtitles, a difference in image qualities might occur between an image having subtitles and an image having no subtitles even in the same scene.
Furthermore, there are many cases where the character area included in the input image employs a luminous value which is significantly different from that of the image area so as to clarify the characters, and the luminous value appears as an outstanding shape in the luminous histogram. Therefore, when the amount of correction is calculated on the basis of the luminous histogram, erroneous correction of image quality might be carried out, resulting in degradation in image quality due to the correction.